1. Field of the Invention
The present invention relates to a device for positioning machine tools and work-pieces used in various types of machining operations. In particular, the present invention relates to a collet-stop assembly used with a collet to fix the position of the machine-tool or work-piece within a lathe, a milling machine, or similar machining equipment. More particularly, the collet-stop assembly of the present invention provides greater ease of set-up, while being considerably smaller than the collet stops hitherto available. Still more particularly, the greater ease of use of the collet-stop assembly of the present invention comes from the minimal number of tools that are needed to install it in a collet, and the very small extension of the collet-stop assembly inside and outside of the collet
2. Description of the Prior Art
Lathes, screw machines, milling machines, and the like universally use collets in many stages of their operation, the collets serving to grip machine tools and work-pieces ("work-items") during machining operations and to properly present them to the point of cutting. The collet is intended to firmly lock the work-items in place during the machining operation, which in general involves rapid spinning of the work-item and/or the application of significant torque to it.
Work-items come in widely varying sizes and shapes, whereas the machines are limited to a relatively small number of sizes and, because of their expense, are even more limited in size within a given shop. The consequent disparity between work-item size and shape, on the one hand, and the dimensions of the work-item-accepting openings of the machines, on the other, is partially overcome by supplementing the collets with a "collet-stop," to be referred to as a "collet-stop assembly" in this discussion. The collet-stop assembly contains a stop-unit, a rod- or tube-like element that is placed along the axis of the collet and that, by virtue of having a flat terminus at its end closer to the "front" of the collet, provides a platform against which the work-piece or tool may be butted. The position of this stop-unit is moved proximally or distally to accommodate work-items of various sizes, ensuring that the proper portion of the item is brought into the zone of work. ("Proximal" refers to the direction along the collet axis toward the front of the collet; "distal" refers to the opposite direction, that is, away from the front--toward the back--of the collet.)
The use of a collet-stop is particularly important when a large number of work-pieces are to be machined and it is necessary to quickly move pieces in and out of the collet while ensuring that the part of each individual work-piece presented to the machining point is unchanged in spite of the removal of one work-piece and the insertion of another. This application puts a premium on the constancy of the stop-unit location in the presence of the stresses imposed by the operation of the machines in question.
A premium is also placed on the ease of adjustment of the stop-unit location. This is important under the opposite of the circumstance just described. That is, when a sequence of work-items of varying sizes must be mounted for machining, one wishes to be able to move the stop-unit proximally and distally simply and dependably while using a minimal number of tools.
Currently available collet-stop assemblies are deficient with respect to both of the characteristics just listed, that is, with respect to dependability of the stop-unit position with time and with respect to the ease of adjusting the stop-unit position. The oldest--and still most common--type of collet-stop assembly uses a stop-unit that is threaded into a sleeve, the sleeve and associated parts of the collet-stop assembly then being affixed to the back of the collet, generally by being threaded tightly into the back of the collet. This threaded stop-unit is moved distally or proximally by rotating it with respect to the sleeve. When it reaches the proper position it is secured against further rotation by tightening one or more lock nuts co-axial with the sleeve and the stop-unit. This design has several major flaws, the most serious being present even when the collet-stop assembly is built to the tightest of specifications. For example, the number of tools one must have available and use is burdensome, starting with the necessity of tightening the assembly to the collet with a wrench to ensure that it does not back off (unscrew) from the collet while the machine is in operation, shifting the stop-unit distally. Then, when one needs to change the stop-unit position it is necessary to hold the collet-stop assembly so that it does not unscrew while the lock-nut is being unscrewed, another wrench or similar device being needed on the lock-nut itself. Then, after rotating the stop-unit so as to advance or retract it, the operator must use a wrench to again tighten the lock-nut.
A separate but related drawback to this common prior-art device is the distance that it extends out the back of the collet, a distance referred to as its "external profile." Not all modern machines have collet-accepting-and-gripping turrets or the like capable of accepting the large external profile that is inherent in this collet-stop assembly's design. (The most common placement will be in a spindle or turret holder of a lathe or similar rotary machining equipment (not shown); however the assemblage will also be used in connection with milling machines, where the collet will be oriented vertically on the machine table, instead of horizontally as with a lathe. In whichever position it is used, it is beneficial that the collet-stop assembly occupy a relatively small distance longitudinally within the collet.) It is inherent because this prior-art design requires that that portion of the assembly external to the collet must provide the means to loosen and tighten one or more lock-nuts and to tighten the assembly onto the collet.
In addition to the drawbacks inherent in this common prior-art design, the fact that it uses threaded stop-units makes it also vulnerable to a common type of manufacturing defect. If the threads on the stop-unit are not concentric with the axis of the collet-gripping means of the machine, mis-alignment of the work-piece or machine-tool--and consequent failure of the machined piece to meet specs--can occur. The same result follows from another common manufacturing defect: the failure of the lock-nut that is used to secure the threaded stop-unit within the collet to be squared off exactly at its face. Furthermore, if the lock-nut is not squared off at its face but is square to the threading of the stop-unit, the stop-unit will be pulled to one side within the collet as that lock-nut is tightened.
Some of the problems associated with the above-described prior art can be resolved by replacing the threaded stop-unit with one that has a smooth, circular external wall. Prior-art collet-stop assemblies do exist that use this approach. These include, for example, the collet-stop assembly of Peterson (U.S. Pat. No. 5,050,896), which uses a smooth rod as its stop-unit and a single set-screw that impinges the stop-unit essentially perpendicularly to its axis as the means to hold the stop-unit in place. Unfortunately, the single set-screw has a tendency to loosen, allowing the (unthreaded) stop-unit to move axially--an unacceptable situation with the high-speed, rotating machining equipment. Moreover, and as is well-known, set-screws tend to cause a scoring or imprinting of the surface of the rod that they are intended to hold in place. In the present context, where the stop- unit tends to be set at a number of nearby positions, the result is that the set-screw may be screwed onto the stop-unit rod's surface at a point partially overlapping an imprint made by an earlier "setting." Subsequently, the rod's prior imprint may "jump" into or away from the current set-screw position, moving the stop-unit and work-item in the midst of the machining operation. The single-set-screw approach presents a further problem in that it tends to push the stop-unit to one side, offsetting the work-piece. This clearly affects the degree of precision for which the machine can be counted on.
In order to avoid having to screw the collet-stop assembly onto the collet, some prior-art devices include a retaining sleeve on the shaft of the assembly, a sleeve that squeezes outward forming a pressure contact with the inner wall of the collet, thus serving to immobilize the assembly with respect to the collet. Typically these sleeves are made of a flexible material, which is undesirable in applications where the goal is to machine pieces to extremely tight tolerances, since it can easily lead to a loosening of the assembly or in movement of the collar-stop and thus the work-piece. Furthermore, under machining conditions it is common for metal chips to be dispersed throughout the equipment, including the collets. When these metal chips become embedded in the relatively soft non-metallic sleeve materials, they render such sleeves useless in later operations.
Apart from everything else, a near-universal problem with prior-stop devices is their sheer complexity. Even disregarding set-up inconvenience one must be concerned because of the increase in cumulative work-piece positioning error associated with the collet-stop assembly being formed of a large number of components. Furthermore, and as alluded to above, the plurality of attachment mechanisms, a plurality that may be distributed among several locations on the prior-art collet-stop assemblies slows the machining operation and increases the likelihood that one of the attachment mechanisms is not completely secured.
Therefore, what is needed is a simple collet-stop assembly requiring minimal effort to affix it to a collet so as to ensure that its stop-unit is held firmly in place with respect to the collet during machining operations. What is further needed is such a collet-stop assembly that also requires minimal effort when the operator needs to adjust the stop-unit location. What is yet further needed is such a collet-stop assembly that contains a minimal number of separate parts, one that does not use a threaded stop-unit, flexible means for coupling to the collet, or any means for coupling to the collet that would put uneven stress on the collet or the collet-stop assembly itself. Finally, what is needed is such a collet-stop assembly that has smaller external and internal profiles than the prior art devices and yet is fully compatible with presently-existing, commonly-used collet assemblies.